This invention relates to anisotropic hollow fiber membranes having immobilized thereon catalysts, preferably biological catalysts, and methods of using such membranes to perform catalytic reactions. Alternative methods also provide for ultrafiltration of a substrate solution while concurrently providing for the catalytic reaction of the substrate.
Hollow fiber membranes have heretofore found widespread use in the fields of ultrafiltration, reverse osmosis, and related separation processes. Ultrafiltration is a process of separation whereby a solution, containing a solute of molecular dimension significantly greater than the molecular dimensions of the solvent in which it is dissolved, is depleted of the solute by being subject to such pressure that the solvent is forced to flow through a membrane. "Ultrafiltration" is the term preferably used to describe such pressure-activated separations involving solutions of solutes of from about 500 molecular weight and above; the term is also conveniently used for processes involving, instead of dissolved molecules, colloidal-sized particles.
"Reverse osmosis" is a term conveniently reserved for membrane-separation processes wherein smaller molecules are involved, for example those molecules or solids which are of a size within one order of magnitude of those of the solvent.
The particular advantages of such membrane-modulated separation processes as described above lie in their potential speed, mild operating conditions and low operating cost compared to various other separation processes such as evaporation, dialysis, ultracentrifugation, chemical precipitation, and the like. These advantages become especially critical when thermally unstable or biologically active materials are to be processed or when relatively large volumes of solvent are present in a solution to be processed.
Successful membrane-modulated separation processes depend, in major part, upon the characteristics of the membrane utilized. Among the desired characteristics are:
(1) High hydraulic permeability to solvent: The membrane must be capable of transmitting liquid at high rates per unit membrane area under modest pressures.
(2) Sharp "retention-cut-off": the membrane should be capable of retaining completely, or very nearly completely, all solutes of a molecular weight (or size) above some first specified value and of allowing the passage of all solutes of a molecular weight (or size) below some second value which should be as close as possible to the aforesaid first value.
(3) Good mechanical durability under the chemical and thermal conditions of service. Most preferably, a membrane should be suitable for use in a wide range of chemical and thermal environment.
(4) A minimum dependence of solvent permeability upon the type or concentration of solute.
(5) High fouling resistance.
To fulfill many of the above preferred criteria desirable for separation membranes, the so-called "anisotropic" membrane has been developed in recent years. (See, for example, U.S. Pat. Nos. 3,615,024; 3,526,588; 3,556,305; 3,541,005; and 3,549,016; all of which are hereby incorporated herein by reference to be generally illustrative of the types of anisotropic membranes contemplated and their method of use.) Briefly stated, the anisotropic membranes useful by this invention are fluid permeable materials characterized by unusually high hydraulic permeabilities through substantially permanent microscopic pores, surprising fouling resistance, excellent retention cutoff characteristics, and having excellent physical strength.
An anisotropic hollow fiber membrane is illustrated in FIG. 1 which is an radial crosssection of a single fiber 4. In the figure, the open channel or lumen 3 through which the liquid to be ultrafiltered is "normally" (but not invariably) passed is defined by a thin film or skin 1 which acts as the active membrane surface. The skin comprises a very thin barrier layer of fine pore material integral with a less dense support outer layer or sponge 2 which is much more porous and provides virtually no increase in resistance to hydraulic flow through the fiber. Various synthetic polymers are used to make the hollow fiber membranes. Unlike other ultrafiltration membranes, hollow fiber membranes are self-supporting, which allows the permeation of materials in either direction, i.e., from the lumen to the outside ("normal mode") or from the outside into the lumen ("backflush mode").
The dimensions of hollow fibers available vary greatly, depending largely on the intended type of separation, pressure drop, flow rates, materials to be separated, etc. For example, ultrafiltration (UF) fibers have lumen diameters of 0.008 inches up to 0.045 inches and higher whereas a common lumen diameter of 0.0016 inches may be employed for reverse osmosis. A typical anisotropic fiber may have a skin thickness of 0.001 mm. attached to a 0.1 mm. layer of open-celled sponge.
Due to the narrow inner channels of the fibers (lumen) liquids containing solute and/or colloidal materials pass through the channels at very high velocity, minimizing solute concentration at the membrane surface due to high shear forces and thereby avoiding blockage of the membrane pores.
In the application of hollow fibers to industrial processes, the fibers are typically combined into, e.g., bundles of 3,000, yielding 30 ft..sup.2 of membrane area. The bundles are inserted into cartridges with both ends rigidly secured in silicone rubber, epoxy or other suitable material. Individual cartridges are arranged in series or in parallel, with appropriate manifolds and pumps to make up custom built systems of desired capacity.